A CDMA wireless network provides a communication link between callers on the Public Switch Telephone Network (PSTN) and callers on mobile stations (MSs). The PSTN supports Pulse Code Modulated (PCM) speech signals, which are digital speech signals sampled at a frequency of 8 KHz. The CDMA network comprises a Base Station Controller (BSC) for compressing groups of 160 PCM speech samples from the PSTN into 20 ms vocoded frames, and a Radio Base Station (RBS) for modulating the vocoded frames into spread-spectrum signals and broadcasting the spread-spectrum signals to the MSs.
The RBS broadcasts the spread-spectrum modulated frames to the MSs at specific frames offset times, which are typically spaced 1.25 ms apart and are disciplined to Global Positioning system (GPS) time. A problem arises in that the PSTN operates asynchronously to GPS time. This problem usually manifests itself as audible shot noise (soft pop or click) which occurs when a PCM speech sample is corrupted, dropped or repeated as a result of time drift between “PSTN time” and “GPS time”. The severity of the audible noise depends on how frequently it occurs and how much discontinuity it introduces.
In first and second generation CDMA radio networks, the BCS repeats or drops a PCM speech sample whenever “PSTN time” drifts from “GPS time” by 125 microseconds, which equals the time period of one PCM speech sample. A drawback of this approach is that it frequently introduces audible noise into the speech signal whenever “PSTN time” and “GPS time” drift by 125 microsecond. In addition, this approach requires providing a highly accurate GPS timing source to processor boards in the BSC, which perform the dropping and repeating of PCM speech samples.
Therefore, there is a need for a BSC that only drops or repeats PCM speech samples when the drift between “PSTN time” and “GPS time” exceeds a threshold much greater than 125 microseconds. This would greatly reduce the occurrence of audible noise caused by the drift between “PSTN time” and “GPS time”. There is also a need for a BSC that relaxes the accuracy requirement of the GPS timing source provided to its processor boards. This would allow the use of commercially available low-cost hardware to distribute the GPS timing source to the BSC's processor boards.